The problem is that the compressor clutch has seized. Rather than drain the refrigerant and remove the compressor I've been trying to find another compressor that I can get refurbished and simply swap in.
The part number for mine is 38810P13J03 (confirmed with Honda UK) but everybody who says they have one has one like this:

They don't have the same part number and are marked for HFC134a refrigerant.
I must admit to being puzzled because they are off 4G preludes. Mines a '94 2.3 btw. Am I right in thinking that these are definitely not what I'm looking for and I should keep looking?
Appreciate any help/advice.